Regarding Marcus Atilius Regulus

[Paullus Scipio]

And while the Periochae are clearly not the same as having Livy's actual text, they do follow the surviving books reasonably closely suggesting that Livy did in fact spend an entire book almost exclusively on the Regulus narrative. Again Livy is a relatively late source (200 years after the fact) but the length and depth of the narrative does suggest a very strong tradition for the Regulus narrative in the later Latin annalists, and clearly by the 1st century BC. Not conclusive proof of Regulus' return after capture etc... but pretty solid evidence!

As for it's absence in Polybius, while this might indicate the story was relatively late in origin (more than one scholar has suggested this), it might also simply be the result of Polybius' purpose in writing. He was not writing a universal history of Rome but a treatise on the rise of Rome, and as such he doesn't mention a lot of things!
Regarding 'historians' mentioning Regulus, the entire narrative does seem to have been in Livy as well...

These points are meritorious, and well put and I would not doubt that a version of the 'Regulus legend' was referred to in Livy, drawing on Annalists and other traditional Roman sources such as family histories rather than earlier historians such as Polybius, but, signifantly this version of the tale seems to include fable-like religious portents/omens such as the slaying of a giant serpent, hinting that the ultimate source is a similar/same embroidered/fanciful tale to the source Horace may have used, with his emphasis on religion. As I have conceded previously, I would agree with you that by Horace and Livy's time(59 BC-17AD), the 'Regulus legend' existed, but it should be remembered that not only Polybius, but Diodorus too, writing just before Horace and Livy, makes no mention of it.

Significantly also, putting together all these clues referred to in this thread points to the 'legend' having originated much later, and almost certainly from a 'family tradition', or possibly a late annalist, a conclusion shared by many scholars, as you point out.

As I said earlier, the clues to this conundrum are all there, and it will be for the reader to weigh them up for him/herself, and reach their own conclusion.... D lol: